The present invention relates to electromagnetic shielding for shielding electromagnetic pickups, other types of electronic equipment, and specific regions of space from electromagnetic radiation, and more particularly to active electromagnetic shielding for providing an electrical cancellation signal for canceling electromagnetic radiation or canceling the response of an electronic device to electromagnetic radiation.
It has long been known that voltages are induced in all conductors exposed to changing magnetic fields regardless of the configuration of such conductors. Electromagnetic radiation will induce electrical signals in electronic devices according to the laws of magnetic induction. Thus it has been desirable in some applications of electronic instrumentation to reduce the inductive noise caused by electromagnetic radiation.
A common method for providing electromagnetic shielding involves the use of passive electromagnetic shielding. A passive shield consisting of layers of high and low permeability material may be used to attenuate electromagnetic radiation passing through it. However, this passive electromagnetic shielding adds substantial bulk and weight to the system that it shields.
Another method for providing electromagnetic shielding is to utilize cancellation coils for generating a canceling electromagnetic radiation in opposition to incident radiation produced by external sources in order to cancel the effects of the incident radiation. In U.S. Pat. No. 5,066,891, Harrold presents a magnetic field sensing and canceling circuit for use with a cathode ray tube (CRT). Magnetic flux gate sensors provide output signals that are functions of detected fields. These signals are then used to control the current in cancellation coils that produce a cancellation magnetic field. Harold explains that it is of great importance that the CRT in a color monitor be protected from the effects of external magnetic fields, and, in particular, time-varying magnetic fields. However, this method provides no compensation to the frequency-dependent amplitude and phase responses of the sensor that picks up incident electromagnetic radiation and the device that generates the cancellation radiation.
Likewise, in U.S. Pat. No. 5,132,618, Sugimoto shows a magnetic resonance imaging system that includes active shield gradient coils for magnetically canceling leakage fields that would otherwise produce eddy currents in the heat-shield tube.
A common method for providing shielding to an electromagnetic pickup is to utilize identical pickup coils connected in series or in parallel so as to cancel the effects of uniform electromagnetic radiation. Pizzarello shows such a system in U.S. Pat. No. 5,045,784 for reducing inductive noise in a tachometer coil. An electric tachometer is a coil of wire that may be attached to a moving part of a motor that passes through a stationary magnetic field. The motion of the wire through the magnetic field induces a voltage that is indicative of the motor's speed. However, if the motor is powered by electricity, changes in the current powering the motor will cause a magnetic flux that also produces a voltage in the coil. Pizzarello shows a stationary pickup coil that is responsive to magnetic flux, and a means for subtracting the pickup voltage from the tachometer voltage.
Likewise, in U.S. Pat. No. 4,901,015, Pospischil shows a cancellation circuit for canceling the response of a magnetic pickup to ambient electromagnetic fields. Pospischil describes first and second pickups that are positioned in parallel with the wavefronts of an interfering electromagnetic field. With such placement, the electromagnetic field impinges simultaneously upon the first and second pickups. The pickups are connected in opposition. Thus, simultaneous impingement of the electromagnetic field upon the pickups is expected to produce a 180-degree phase displacement of the received signals.
If the electrical path lengths of the received signals in Pospischil's cancellation system are different where they are combined (summed), the relative phase difference between the received signals will not have 180-degree phase displacement. Thus, the signals will not cancel. Pospischil shows that differences in the electrical path length occur when the propagation path lengths of the signals received by the pickups are different (e.g., the signals do not impinge upon the pickups simultaneously). These differences in propagation path lengths can result from reflections, multipath delay, superpositions of multiple received signal components, or received electromagnetic signals having non-perpendicular angles of arrival.
Pospischil does not identify nor compensate for electrical path-length differences (e.g., differences in impedance) that occur between different electromagnetic receivers (pickups). Such pickup assemblies are also used with electric guitars and are known as "hum-bucking" pickups. This technique is not effective for providing a high degree of cancellation because slight differences between the pickups, even pickups that are substantially identical, cause the frequency-dependent amplitude and phase response of the pickups to differ significantly from each other. Thus the pickup signals will not be exactly out of phase and equal in amplitude when they are combined.
A prior-art method for providing shielding to an electromagnetic pickup from an electromagnetic source that produces a non-uniform field is to "unbalance" either the pickup device or the electromagnetic source. Such a method is described by Hoover in U.S. Pat. No. 4,941,388. Hoover uses amplitude-adjustment techniques to compensate for amplitude variations between the responses of separate pickups to electromagnetic radiation generated by an electromagnetic sustaining device that drives the vibrations of a string on an electric guitar. However, Hoover does not compensate for differences in the pickup coils which cause the amplitude-variation of the responses of the pickups to be frequency-dependent. Thus, Hoover's proposed solution results in poor cancellation over a broad range of frequency. Furthermore, Hoover does not compensate for phase-variations that occur between different pickup coils. The resulting cancellation from the unbalancing method is poor.
Hoover describes the operation of negative feedback in a system where a magnetic pickup provides an electrical signal to a magnetic driver that generates an electromagnetic field to which the pickup responds. Hoover mentions that the system tends to drift from the negative feedback condition at higher frequencies, and identifies the cause of this drift as distortions in the phase-response of the system resulting from the pickup, driver, and amplifier in the system. Hoover does not present an effective method for controlling the phase-response of the system, nor does Hoover present the mathematical relationships between phase and frequency resulting from the driver and pickup coils. Rather, Hoover proposes the use of a low-pass filter to reduce the gain of the system at which the negative feedback condition breaks down.
Methods of active phase-compensation are described by Rose in U.S. Pat. No. 4,907,483, U.S. Pat. No. 5,123,324, and U.S. Pat. No. 5,233,123. Rose uses active circuits for determining the frequency or frequency range of an electrical signal from an electromagnetic pickup. Active phase-adjustment is applied to the pickup signal, which is used to power an electromagnetic driver that generates an electromagnetic driving force on a vibratory ferromagnetic element of a musical instrument. The purpose of the phase-adjustment of the pickup signal is to provide a driving force to the vibratory element that is substantially in-phase with its natural motion. Because the purpose of Rose's invention is to improve the efficiency of the electromagnetic drive force on the element, it is apparent that a passive phase-compensation circuit would be preferable to Rose's active phase-compensation circuit. However, Rose does not realize the mathematical relationships between phase and frequency that provide the basis for constructing a passive phase-compensation network. Furthermore, Rose's invention does not provide simultaneous phase-compensation to more than one harmonic.
Another method for providing electromagnetic shielding is to orient the angle of a pickup coil to incident electromagnetic radiation such that the electrical current induced in the coil by the electromagnetic radiation will substantially cancel. One application of this method is shown by Burke in the Handbook of Magnetic Phenomena, published in 1986. Burke uses a transmitting coil that produces electromagnetic radiation and a receive coil that senses radiation. The two coils can be configured in such a way that no energy is transferred between the transmitting and receiving coils. Burke shows the receiving coil oriented with the axis of its turns at right angles to the direction of the magnetic field produced by the transmitting coil. Burke explains that the instantaneous generated voltage of the receive coil is determined by the instantaneous rate of change of the magnetic flux passing through the coil. If the flux is directed at right angles to the coil's axis, none of it is intercepted by the coil, and the instantaneous rate of change through the coil is zero. This method of cancellation was used in an electromagnetic sustain device for electric guitars marketed by T Tauri Research of Wilmette Ill. in November, 1988, and patented by Tumura, European Patent Application No. 92307423.1 filed on Aug. 13, 1992, and U.S. Pat. No. 5,292,999. The actual effectiveness of this technique is limited by several factors, such as the uniformity of the pickup coil's windings, the uniformity of the electromagnetic radiation near the pickup, interference due to other nearby conducting materials, and the difficulty of precisely positioning a pickup coil in a field whose intensity varies as the inverse square of the distance from its source.
Another method for providing active electromagnetic shielding is the differential transformer also shown by Burke. The differential transformer comprises a drive coil for generating a magnetic flux, and two pickup coils wrapped around a ferromagnetic core that includes a moveable armature that, when moved, varies the reluctance of the magnetic path associated with each pickup coil. If the two pickup coils are identical, and if the two magnetic paths about which they are wound are identical, the voltages induced in each pickup coil will be the same. However, Burke explains that the two pickup coils nor the two magnetic paths can be made exactly the same, therefore a differential transformer will always have some output voltage under zero stimulus.
Coils of wire whose currents support magnetic fields in space function as antennas radiating electromagnetic energy. There are several cancellation methods used with antennas that act as electromagnetic shielding. One of these methods is the basis of operation for a sidelobe canceller that uses an auxiliary antenna in addition to a main antenna. Combining the outputs from the two antennas results in cancellation of the antenna beam pattern in the direction of a noise source so that the effective gain of the antenna in that direction is very small. Likewise, the multiple sidelobe canceller addresses the problem of multiple noise sources.
Delay-line cancellers are used in systems where multiple radar pulses are transmitted. These cancellers are used to detect moving objects. In a single-element delay-line canceller, a received pulse is delayed and added to another pulse received later so that the pulses reflected by stationary objects are out of phase and thus cancel, whereas the pulses reflected by moving objects do not cancel.
Several methods are used to allow an antenna to simultaneously transmit and receive electromagnetic radiation. For example, in a continuous wavelength radar system, a single antenna may be employed since the necessary isolation between transmitted and received signals is achieved via separation in frequency as a result of the Doppler effect. The received signal enters the radar via the antenna and is heterodyned in a mixer with a portion of the transmitted signal to produce a Doppler beat frequency.
An intermediate-frequency receiver may use separate antennas for transmission and reception. A portion of the transmitted signal is mixed with an intermediate frequency, and then a narrow-band filter selects one of the side bands as the reference signal, which is mixed with the signal from the receiver antenna.
It is one object of the present invention to provide active electromagnetic shielding for canceling the effects of electromagnetic induction in electrical circuits. It is a related object of the present invention to reduce interference between transmitters and receivers of electromagnetic radiation that operate simultaneously. It is another object of the present invention to provide a cancellation circuit that allows a single antenna element to simultaneously transmit and receive electromagnetic radiation. It is still another object of the present invention to compensate for frequency-dependent amplitude and phase responses of electromagnetic receivers and transmitters.